The object of this invention consists of a process for separating metal compounds, even in small percentages, from dusts and sludges, by reduction.
This process is particularly suitable for separating vanadium pentoxide from the ashes derived from the combustion of heavy fuels, to obtain very pure vanadium at an extremely low cost.
The reduction process may be equally well applied to the separation or passivation of hexavalent chromium compounds, which are highly polluting substances, and which, if present in relatively high percentages, may be recovered, or, if present only in limited percentages in the material to be treated, may be reduced to trivalent chromium by the same reduction process, and then passivized to transform them into compounds insoluble in water.
It is known that vanadium is a metal present in many metal alloys, and in the form of vanadium pentoxide it is widely used as a catalyst, in the place of the more expensive platinum sponge.
Vanadium is present in most crude oils, in a few tens of parts per million, varying considerably according to the origin of the crude oil.
Vanadium is therefore present in general in the ashes derived from the combustion of the heavier crude oil fractions. Extraction of vanadium from these ashes by traditional methods generally does not pay, in view of its low percentage; in addition, this vanadium is always accompanied by other metals from which it has to be separated by a second stage, making the production costs even higher.
As per Canadian Pat. No. 752.773, the vanadium present in the ashes obtained from the combustion of the heavier fractions of crude oil is separated as follows:
Diluted sulphuric acid is added to the dust obtained from the ashes of heavy fuel, so as to obtain vanadium sulphate, nickel sulphate, magnesium sulphate and calcium sulphate; after 2 to 3 hours the reaction has ended, and then hot water is added so as to dissolve all the sulphates into a solution.
This first stage of the process is common to all the processes, including the process on which this invention is based. The solid fraction is separated by filtering, and using suitable methods the washing water is separated from the concentrated solution of sulphates, to be called hereinafter the "filtrate".
As per the above-mentioned Canadian patent, the filtrate thus obtained, consisting of a concentrated solution, is oxidized, to transform all the vanadium into pentavalent vanadium.
Oxidation is preferably carried out in a diluted water solution, using sodium chlorate, potassium chlorate or hydrogen peroxide as the oxidizer.
Sodium chlorate is preferable as the oxidizing agent, being readily available on the market and being less expensive than potassium chlorate or hydrogen peroxide.
Oxidation is best carried out at a temperature of about 150.degree. F. (about 66.degree. C.), although the reaction can take place, more slowly, even at lower temperatures. At temperatures higher than the above there is a loss of the oxidizing agent (oxygen) caused by the excessive development of chlorine gas.
During oxidation the pH of the said solution need not be an exact value, but may vary within a certain range. When the oxidation process is complete, the colour of the solution turns from blue to green or yellow.
Once oxidation has been completed, the solution is submitted to another chemical treatment. This latter treatment consists of the precipiatation of the vanadium from the solution by adding a suitable reagent, for example ammonia, preferably anhydrous. Precipitation is effected at a temperature between 180.degree. F. (82.22.degree. C.) and 200.degree. F. (93.33.degree. C.) and at a pH between 1.7 and 2.1.
Experts in this field know that the vanadium pentoxide may be precipitated at various temperatures, but due to the nature of the chemical reaction involved, it is preferable to operate as close as possible to the boiling point of the solution. The higher the temperature, the greater the result and the shorter the time required to achieve complete precipitation. As far as concerns the best pH, at a low pH level the vanadium outturn is lower, while with a pH of about 2, the iron present in the solution may precipitate in considerable quantities, which is not desirable. In addition, with a high pH the process requires excessive amounts of chemical reagents, and consequently entails higher costs. In any case, according to Canadian Pat. No. 752.773, the transformation of the vanadium into vanadium pentoxide is made critical by the danger of precipitating the iron in the solution. If this occurs, it is necessary to recycle the product just obtained.